Guests - If You want access to member only forums on HSO. You will gain access only when you Sign-in or Sign-Up on HotSpotOutdoors.

It's easy - LOOK UPPER right menu.

  • Announcements

    • Rick

      Members Only Fluid Forum View   08/08/2017

      Fluid forum view allows members only to get right to the meat of this community; the topics. You can toggle between your preferred forum view just below to the left on the main forum entrance. You will see three icons. Try them out and see what you prefer.   Fluid view allows you, if you are a signed up member, to see the newest topic posts in either all forums (select none or all) or in just your favorite forums (select the ones you want to see when you come to Fishing Minnesota). It keeps and in real time with respect to Topic posts and lets YOU SELECT YOUR FAVORITE FORUMS. It can make things fun and easy. This is especially true for less experienced visitors raised on social media. If you, as a members want more specific topics, you can even select a single forum to view. Let us take a look at fluid view in action. We will then break it down and explain how it works in more detail.   The video shows the topic list and the forum filter box. As you can see, it is easy to change the topic list by changing the selected forums. This view replaces the traditional list of categories and forums.   Of course, members only can change the view to better suit your way of browsing.   You will notice a “grid” option. We have moved the grid forum theme setting into the main forum settings. This makes it an option for members only to choose. This screenshot also shows the removal of the forum breadcrumb in fluid view mode. Fluid view remembers your last forum selection so you don’t lose your place when you go back to the listing. The benefit of this feature is easy to see. It removes a potential barrier of entry for members only. It puts the spotlight on topics themselves, and not the hierarchical forum structure. You as a member will enjoy viewing many forums at once and switching between them without leaving the page. We hope that fluid view, the new functionality is an asset that you enjoy .
Sign in to follow this  
PTocko

How much power (if any) do you lose when it gets warm???

Recommended Posts

Question - how much power do boat engines lose when the temperature goes up? I know with snowmobiles that a 20 - 30 degree difference in temp can make a huge difference on performance - even on the new FI models that "compensate" for temperature. Reason I ask is I've been very happy with how my motor has been dialed in, ran it yesterday (1st really "warm" day I've been on the water)and it has "lost" a couple hundred RPM's. Everything else - weight, etc - is the same. Motor was running fine but I'm wondering if something else is going on that I should be concerned about. FYI - Motor is a 1999 Mercury 40 4 stroke (carb'ed), boat is a 2001 Alumacraft Navigator 165. Propeller is a Cabellas SS 10 1/2 x 11. Any help is appreciated. Paul

Share this post


Link to post
Share on other sites

Your motor is water cooled, from the lake (an unlimited supply of cold coolant), so I wouldn't expect that much of a drop in RPM's due to air temperature.

marine_man

Share this post


Link to post
Share on other sites

Cold water is denser than warmer water. That's one of the reasons boats run faster in cold/cool water. As the summer progresses and the lakes warm up your boat will slow down a bit.

Share this post


Link to post
Share on other sites

Wait a minute guys, The original question is valid in relation to the temperature of the air and it's corresponding decrease is density with the higher temp. While the water cooling will keep the block at a constant temperature and have a small moderating effect, it's the air density with relation to fixed carb jetting and the subsequent fuel/air mixture that causes the decrease in horsepower output. As the air warms up, it's molecules spread apart, resulting in less weight (mass) per unit of volume. Since a carburetor is a fixed restriction to the volume of air flow (not mass) and it's jets limit fuel flow in the same way providing a constant mass, as the air gets less dense the mixure gets richer.

This change in the stochiometric mixture or air/fuel ratio is what causes the reduction in power output. Ideally, with gasoline as the fuel, the ideal ratio is around 14.7:1 by weight of air to fuel, so you can see as the air becomes less dense the mixture will richen and decrease power. The output curve also drops to the lean side of 14.7:1 with the added problem of increased combustion temperature which can result in burned pistons and other bad stuff. Oxygenated fuel (ethanol) will run better with less air density due to the fuel characteristics. Since the manufacturer usually jets the engine to the rich side to reduce warranty claims, and that jetting most likely done around sea level where the air is most dense to begin with, the rise in air temperature will have a noticeable effect on performance.

Share this post


Link to post
Share on other sites

Wait a minute guys, The original question is valid in relation to the temperature of the air and it's corresponding decrease is density with the higher temp. While the water cooling will keep the block at a constant temperature and have a small moderating effect, it's the air density with relation to fixed carb jetting and the subsequent fuel/air mixture that causes the decrease in horsepower output. As the air warms up, it's molecules spread apart, resulting in less weight (mass) per unit of volume. Since a carburetor is a fixed restriction to the volume of air flow (not mass) and it's jets limit fuel flow in the same way providing a constant mass, as the air gets less dense the mixure gets richer.

This change in the stochiometric mixture or air/fuel ratio is what causes the reduction in power output. Ideally, with gasoline as the fuel, the ideal ratio is around 14.7:1 by weight of air to fuel, so you can see as the air becomes less dense the mixture will richen and decrease power. The output curve also drops to the lean side of 14.7:1 with the added problem of increased combustion temperature which can result in burned pistons and other bad stuff. Oxygenated fuel (ethanol) will run better with less air density due to the fuel characteristics. Since the manufacturer usually jets the engine to the rich side to reduce warranty claims, and that jetting most likely done around sea level where the air is most dense to begin with, the rise in air temperature will have a noticeable effect on performance.

Well if you want to get technical. wink

Share this post


Link to post
Share on other sites

Wait a minute guys, The original question is valid in relation to the temperature of the air and it's corresponding decrease is density with the higher temp. While the water cooling will keep the block at a constant temperature and have a small moderating effect, it's the air density with relation to fixed carb jetting and the subsequent fuel/air mixture that causes the decrease in horsepower output. As the air warms up, it's molecules spread apart, resulting in less weight (mass) per unit of volume. Since a carburetor is a fixed restriction to the volume of air flow (not mass) and it's jets limit fuel flow in the same way providing a constant mass, as the air gets less dense the mixure gets richer.

This change in the stochiometric mixture or air/fuel ratio is what causes the reduction in power output. Ideally, with gasoline as the fuel, the ideal ratio is around 14.7:1 by weight of air to fuel, so you can see as the air becomes less dense the mixture will richen and decrease power. The output curve also drops to the lean side of 14.7:1 with the added problem of increased combustion temperature which can result in burned pistons and other bad stuff. Oxygenated fuel (ethanol) will run better with less air density due to the fuel characteristics. Since the manufacturer usually jets the engine to the rich side to reduce warranty claims, and that jetting most likely done around sea level where the air is most dense to begin with, the rise in air temperature will have a noticeable effect on performance.

This is what I was looking for. I agree the engine operating temp would not change much, I was talking about the different effect warmer AIR would have on the performance. In snowmobiles you jet down (leaner) as it gets warmer to maintain power - if you don't you will lose power. I've never changed the jets in my boat so I am assuming that I am losing power from those 40 degree mornings to the 85 degree days. On top of that if they are jetted rich to begin with you could be leaving lots of power on the table. I guess not 100% definiative in diagnosing my "strange" loss of RPM's but it sounds like I am not thinking totally crazy. Thanks, Paul

Share this post


Link to post
Share on other sites

Good post Hydro... I still think dropping 200 rpm seems like a lot though...

marine_man

Maybe, maybe not. Your only talking about a reduction in RPM's of 3.44% which on a 40 Hp motor MAY only mean a redcution in 1.34 hp (not sure of the EXACT relationship between RPM's and HP but I'm sure they're tied together). If you are not jetted right on a snowmobile you could easily lose 10% - 20%. MM - please note I am not saying this IS my problem, I'm just saying when you look at the numbers it really isn't THAT much. Paul

Share this post


Link to post
Share on other sites

for my snowmobile it will hit 75 when its -30 when its 30 it will only go 50 and takes a ton of gas.. i think the reason fro sleds dropping power is the wetter snow causing more friction

Share this post


Link to post
Share on other sites

for my snowmobile it will hit 75 when its -30 when its 30 it will only go 50 and takes a ton of gas.. i think the reason fro sleds dropping power is the wetter snow causing more friction

Some is friction, A LOT is you do NOT have as much power. You can compare on bare ice (or a grass drag or heck even an asphalt drag strip) - anywhere the friction is relatively "constant". You WILL be faster at -30 versus +30, I guarantee it. Air is denser at -30 which means you can use more gas and still maintain the optimal air to fuel ratio - more gas + denser air = bigger bang!! This is simple physics. If you are not able to go faster at -30, something else is wrong. Paul

Share this post


Link to post
Share on other sites

Hiya,

I think there might be a bit more to this than just what hydro stated. I would agree 100% with what he stated, but one thing I think may add "a little" to the loss in RPMs is that as was mentioned, high temperatures will force molecules apart...both in air and the water itself. Thus...although minimal, the boat will sit lower in the water...even at planing speeds. This, in effect will result in more "wetted surface" of the boat in the water while at speed, and that in itself will mean lower RPM since the motor has to work harder.

I know with my yamaha 90 2 stroke, I lose about 200 rpms when it gets hot as well...worse when it's humid.

Steve

Share this post


Link to post
Share on other sites

Create an account or sign in to comment

You need to be a member in order to leave a comment

Create an account

Sign up for a new account in our community. It's easy!

Register a new account

Sign in

Already have an account? Sign in here.

Sign In Now

Sign in to follow this  



  • Posts

    • Uhhhhh... I for one am at a complete loss for words.
    • Here is a project for you when fishing gets slow.  
    • I've never been to Boone's but I would think that Ahlman's Gun Shop down by Waterville might be similar and another possible resource.
    • Was able to get out these last couple weeks a couple times to enjoy the waterfowl before most of them left for good. Surprisingly was able to find a couple sea ducks on back to back weekends. The harlequin was a first for me in MN and the Long Tail was the second of my life.  Young long tail days before this lake froze over. Hen Harlequin below the St. Cloud darn couple weeks ago. American blackduck - may have some slight mallard mixed in with it as I noticed some green on its head from certain angles. widgeon picture I liked the colors with. This was taken early November.  
    • Hey, I just started mine and did about 5 circles around the yard just to smell some exhaust!
    • Wow. That's like a dictionary.
    • I downloaded and printed the manual. Worth it. It does so many things it would take years to fumble through to find shortcuts and fine tune without it. 
    • Well, the package arrived yesterday and I got it put together last night. Overall impression compared to the lowrance is that this is by far a better unit.   Starting with the bag. The zippers and fabric are more heave duty and look to be able to stand up to much more use without problems.   The plastic holder- probably a wash between the two units but the HB has done some nice things with cord management that give it the nod. Also feels a bit more durable.   Now to the graph- I live the display. It looks more sharp and more professionally designed than lowrance and to be honest I like the look better that the marcum lx units as well. Just personal preference. The adjustments are more intuitive and overall after being a lowrance guy for a long time I can't find a single thing that lowrance did better.
    • Dude, it's ok shes not on here.  Save that for Facebook! 
  • Our Sponsors